The present disclosure generally relates to data processing systems and, more particularly, relates to components of a channel subsystem of a data processing system.
A channel subsystem (CSS) directs the flow of information between input/output (I/O) devices and main storage of a data processing system and is mainly comprised of one or more I/O processors (IOPs) and I/O channel paths (channels) with some participation of the central processors (CPs). IOPs are also synonymously referred to as system assist processors (SAPs).
In a simple data processing system, I/O instructions are initiated by a CP that might execute a sequence of instructions that partly use only the resources of the CP and that partly directly control the resources and the operations of the CSS. In some situations, IOPs perform a portion of an I/O operation, and one or more channels handle other parts of the I/O operation. The IOPs determine which channel to select for an I/O operation. The channels handle the actual data transfers into and out of processor memory and execute commands by forming orders that are sent to the I/O device controllers. The IOP also handles general parts of the I/O operation such as communicating with the CPs through interfaces such as control blocks in reserved memory called hardware system area (HSA), scheduling the I/O operations, and reporting status conditions to a CP. A single IOP can handle requests from multiple CPs and report status to different CPs.